This invention relates generally to controlling recirculation and cavitation in a centrifugal pump at reduced flow rates by reinjecting fluid into the inlet of the impeller of the centrifugal pump.
More particularly, this invention concerns a method and apparatus for transferring fluid from the discharge of the impeller and reinjecting such fluid through an annular reinjection port near the inlet or suction of the impeller. Passageways are used to transfer the reinjection fluid from the discharge to the inlet of the centrifugal pump, and the passageways are shaped so that the fluid will be reinjected into the inlet of the pump in an optimum fashion, imparting a flow direction that produces a minimum angle of attack with respect to the impeller blades.
A wide variety of fluids (including water, hydrocarbons, slurries, air, natural gas, and other liquid and gaseous materials) are pumped using centrifugal pumps.
A centrifugal pump has an impeller 10 mounted on a shaft 11. The impeller 10 and the shaft 11 are rotatably mounted in a housing or casing 12. A motor or other power source (not shown) is used to rotate the shaft 11.
Fluid is pumped through a centrifugal pump by rotating the impeller 10 and the shaft 11. This rotation creates a suction at the inlet 13 of the pump by imparting momentum to the fluid, causing the fluid to travel through the impeller 10 and out the discharge tips 14 into the discharge annulus or discharge header 15. The discharge header 15 is connected to, or in fluid communication with, an output pipe or conduit through which the fluid is pumped. The inlet 13 of the pump is typically connected to a pipe or conduit through which fluid flows toward the centrifugal pump.
Centrifugal pumps have a certain flow rate at which the pump operates most effectively, which is commonly referred to in the art as the best efficiency point. A centrifugal pump can operate effectively only over a limited range of flow rates above and below its best efficiency point. As used herein, flow rate is the rate of flow of fluid through the input pipe connected to the input 16 of the pump.
At low flow rates, the direction of flow of a portion of the fluid at the inlet 13 of the impeller 10 may actually reverse. Flow reversal may also occur at the discharge tips 14 of the impeller 10. Flow reversal at the inlet or at the discharge tips of the impeller in a centrifugal pump is known in the art as "recirculation". All impellers will exhibit recirculation at some reduced flow rate. Depending on the size and speed of the pump, and the fluid being pumped, the effect of recirculation can be very damaging.
If a pump is operated at low flow rates, significant adverse consequences may result due to recirculation. The consequences of recirculation can include cavitation damage to the impeller vanes at the inlet to the impeller, impeller and case erosion, cavitation damage to the vanes at the discharge of the impeller, random crackling noise and noisy operation, shaft deflection and stress, axial movement of the shaft, radial and thrust bearing failures, cracking or failure of the impeller shrouds at the discharge of the impeller, shaft failures, seal problems, surging in the suction of the centrifugal pump, and high vibration at low flow rates. Recirculating fluid can erode metal impeller vanes as if the metal vanes were subjected to constant high velocity sandblasting.
It is inherent in the dynamics of the pressure field that, if the flow rate is reduced, every impeller design must recirculate at some point--it cannot be avoided. A description of the problem of recirculation appears in an article authored by Warren H. Fraser, entitled "Flow Recirculation in Centrifugal Pumps", which is incorporated herein by reference. The reversal of flow of part of the fluid, at the same time that a portion of the fluid is entering the impeller, usually produces vortices which cavitate and produce random sharp crackling noise.
Recirculation at the inlet of a centrifugal pump is sometimes referred to as "suction recirculation."
The reversal of flow of fluid, or backflow, at the pump inlet is believed to occur mainly in the vicinity of the circumference of the inlet pipe wall, with the center of the stream of fluid continuing to exhibit forward flow. In other words, fluid in the center of the input 16 may flow toward the pump, while fluid around the outside, or around the circumference, may reverse and flow away from the pump.
A vortex may form at the inlet of the centrifugal pump due to suction recirculation. A fixed vortex may be produced that travels around with the rotation of the impeller vanes. This vortex will typically cavitate at its core and attack the metal surface of the pressure side of the vanes.
Reversal of flow at the discharge of the impeller may also occur. This is sometimes referred to as "discharge recirculation." Recirculation at the discharge of the impeller may produce a vortex that rotates with the impeller vanes. If the velocities of the reverse flow are of sufficient magnitude, the vortex will cavitate and attack the metal surface of the vanes. Vortices in the inlet of the impeller may possibly induce discharge recirculation.
A third phenomenon, sometimes loosely referred to in the art as "recirculation", involves the flow of fluid from the impeller discharge back to the suction through the wear ring clearances. This should be referred to instead as "wear ring leakage", and may normally occur in any pump.
One way of expanding the range of flow rates over which a centrifugal pump may operate is to provide for the reinjection of fluid from the discharge of the impeller back to the inlet of the impeller. The effective flow through the impeller, which may be referred to as the "apparent flow rate", may be increased, even though the actual flow rate into the input and out the discharge header is low. Thus, the apparent flow rate may be maintained within an acceptable range of operation for the centrifugal pump.
This invention relates to an improved means for reinjecting fluid into the inlet of the impeller of a centrifugal pump to maintain effective operation of the pump at reduced flow rates.
In the past, proposals have been advanced for reinjecting fluid which failed to consider the angle or direction of flow of the reinjected fluid. In order to effectively minimize the recirculation vortex that is formed the fluid must be reinjected at a proper angle and in a proper direction. It is important to direct the reinjected fluid directly into the inlet of the impeller at the proper angle and in the proper direction.
Cliborn's U.S. Pat. No. 2,865,297 illustrates a device which fails to control the direction in which fluid is reinjected. Cliborn fails to utilize the momentum of the fluid which is discharged from the impeller to propel the fluid through the reinjection passageway. Cliborn appears to rely solely on pressure differentials to urge the reinjection fluid through the reinjection passageway. In Cliborn, the kinetic energy of the fluid must be converted to potential energy in the form of pressure, and the fluid must then be reaccelerated toward the input of the pump.
Proposals have been advanced to place an inducer in front of the impeller. Two such proposals appear in U.S. Pat. No. 3,504,986, issued to Jackson, and U.S. Pat. No. 3,723,019, issued to Berman. An inducer cannot be removed when the flow rate reaches an optimum point, and consequentially may interfere with operation of the pump at its most efficient flow rates.
Other patents which may be of interest are U.S. Pat. Nos. 4,149,825; 3,976,390; 3,901,620; 3,741,676; 3,588,266; 3,268,155; and 3,095,820.
The problems of the prior art devices which are discussed above are not intended to be exhaustive. Other problems may exist. The above discussion does indicate that prior art devices have left room for significant and needed improvement.